The present invention relates in general to an improved piezo-electrically tuned optical resonator, and more particularly, to a piezo-electrically tuned ring laser.
Laser-diode pumped monolithic lasers have permitted the construction of frequency stable, single-mode lasers. Single-mode frequency-stable lasers have many applications in coherent communications, laser radar, and in the creation of visible light through harmonic conversion. For all these applications, the ability to quickly tune the oscillating frequency of the laser is of great value.
The tuning of a laser requires that the optical length of the laser resonant cavity be changed. For non-monolithic lasers, a common technique is to use a piezo-electric element to translate one of the mirrors which define the resonant cavity of the laser. This technique is not possible with a monolithic laser. A monolithic laser is defined as having a resonant cavity consisting of a single element, which is made of the active laser material. The resonator mirrors are formed on surfaces of the single element. Thus the mirrors, not being separate elements, cannot be translated in the usual way. Some other technique is needed to change the optical length of the resonant cavity.
In the past, thermal tuning was used for frequency tuning. The thermal expansion of the monolithic element, combined with a thermally-induced change in the index of refraction, lead to the change in the optical length of the resonator required for frequency tuning. Thermal tuning is necessarily quite slow, as thermal time constants of even small objects tend to be on the order of one second. Much faster tuning, with response times less than 1 millisecond, are desired.
It is known that the indicesof refraction of materials change when a stress is applied. It is also known that even the most rigid materials expand or contract when a stress is applied. Stress can be applied to a solid element very quickly, with the only fundamental physical limit being the speed of sound in the material. Application of stress to monolithic lasers thus is a way to quickly change the index and the dimensions and thus the optical length, thus inducing a tuning of the resonant frequency.
Heretofore, stress tuning of a monolithic rod laser has been achieved. In this prior art stress-tuned laser, a monolithic rod of lasant material is held in a clamp structure to apply a substantial bias force and thus stress to the laser rod. A portion of the clamp structure includes a stack of piezo-electric elements. A voltage is applied to the piezo-electric elements to modulate the bias force applied to the monolithic rod laser to tune the laser over a substantial tuning range as of 90 GHz. Such a stress-tuned diode laser is disclosed in an article appearing in Vol. 12, No. 12, of "Optics Letters", pgs. 999-1001 of December 1987.
Problems associated with the prior art stress-tuned laser include the fact that the clamp structure is relatively large. The large size of the tuning structure has two disadvantages. First, the response time of the system is slowed. Second, large size makes the entire laser more sensitive to acoustic noise, reducing the frequency stability of the laser.
It would be desired to obtain a piezo-electrically tuned monolithic laser having decreased response time and improved frequency stability at the cost of tuning over a lower tuning range.